I'm not best friends with the housemate, but think she's an ok girl and we chat when we see each other, but would never go out just the two of us.
Me and boyfriend went out for a couple of drinks on the Friday and walked home and ended up getting a 2for1 pizza deal. We got home and ate one and said we would save the other one for Saturday.
Housemate was in the room with her boyfriend so we said goodnight to them and went to bed.
Boyfriend text me the next day to say that they had eaten the other pizza after we'd gone to bed.
I never said anything to her but I was slightly pissed off about it, because I have room mates and would never eat their food and I especially wouldn't wait until they'd gone to bed to do it either.